Elena M. Bennett is a graduate student in limnology and marine sciences at the university of Wisconsin–Madison, Madison, WI 53706.
You are watching: How do humans impact the phosphorus cycle
BioScience, Volume 51, problem 3, march 2001, Pages 227–234, https://doi.org/10.1641/0006-3568(2001)051<0227:HIOEPA>2.0.CO;2
Human actions—mining phosphorus (P) and carrying it in fertilizers, animal feeds, farming crops, and other products—are changing the global P cycle, bring about P come accumulate in few of the world"s soil. Increasing P level in the soil elevate the potential ns runoff come aquatic ecosystems (Fluck et al. 1992, NRC 1993, USEPA 1996). Making use of a worldwide budget approach, we estimate the increase in net ns storage in terrestrial and also freshwater ecosystems to be at the very least 75% better than preindustrial levels of storage. Us calculated an agricultural mass balance (budget), which shown that a huge portion the this P build-up occurs in agricultural soils. Separate P budgets that the farming areas the developing and also developed nations show the the rate of P accumulation is diminish in arisen nations but increasing in developing nations.
Phosphorus build-up in upland soils may affect freshwater ecosystems. Production in many lakes relies on ns input (Schindler 1977). Overenrichment through nutrients can cause excessive production in lakes, a problematic problem known as eutrophication, in i beg your pardon water quality is impaired (see the box on p. 225). Phosphorus generally enters aquatic ecosystems sorbed to soil particles that room eroded right into lakes, streams, and also rivers (Daniel et al. 1994, Sharpley et al. 1994). Much of this runoff wake up during significant erosion-causing storms (Pionke et al. 1997). Potential P pollution of aquatic ecosystems is therefore strongly influenced by watershed floor use and also the concentration of ns in watershed soils: any type of factor that increases erosion or the lot of ns in the soil increases the potential ns runoff to downhill aquatic ecosystems (Daniel et al.1994, Sharpley et al. 1994).
Phosphorus buildup in upland soils could reason water quality problems in overfill of eutrophication checked out to date. Plenty of years might pass between buildup of ns in soil and the appearance of adverse results in freshwater ecosystems (Reed-Andersen et al. 2000). Such adverse results might appear abruptly if the vulnerability the the mechanism increases gradually until a threshold is happen (Heckrath et al. 1995). Soil p accretion can lead come sudden and unanticipated changes in aquatic ecosystem productivity. The could also cause lags in between management actions taken to regulate eutrophication and also the time when results of those actions are realized (Stigliani et al. 1991).
In this write-up we ask, “Are there changes in the worldwide P bike that could increase results on freshwater systems?” Specifically, we attempt to identify whether over there is, in fact, increased P storage in the soils the terrestrial ecosystems. We deal with this question through synthetic of present literature and use a budget technique to calculation the boost in p storage in upland ecosystems.
Changes in the worldwide nitrogen (N) cycle have been well defined (Vitousek et al. 1997, Caraco and also Cole 1999). Human impacts on the an international P bicycle are much less clear (see, however, Howarth et al. 1995, Tiessen 1995). Nevertheless, instances of human impact on the p cycle leading to accumulation of p in upland systems have been uncovered both locally and regionally. Because that example, Lowrance and colleagues (1985) uncovered that imports of P exceeded exports by 3.7 to 11.3 kg ha−1 yr−1 in four subwatersheds that the little River in the Georgia coastal Plain, with human being fertilizer inputs come the four subwatersheds much exceeding organic precipitation inputs. Similarly, a P budget plan of the top Potomac River basin revealed the over 60% the imported ns was kept within the watershed (Jaworski et al. 1992). In this case, p retention was led to by an excess of fertilizer and also animal feeding inputs end outputs of agricultural products. In a Florida study, Fluck and also colleagues (1992) found that much less than 20% of ns input come the Lake Okeechobee watershed in fertilizers was calculation in farming and various other products. A P budget plan of the Lake Mendota watershed also showed a human-caused increase in p storage. Net ns storage in the 686-square-kilometer watershed was discovered to be over 500,000 kg in 1996 (Bennett et al. 1999). Natural P motion (in dry and also wet deposition and also hydrologic outputs) accounted for less than 5% of all P activity into and out that the watershed.
Some national and local studies reveal comparable human impacts and increased ns storage in terrestrial ecosystems. Runge-Metzger (1995) calculated network fertilization (fertilizer intake minus crop removal) the 0.7 to 57.2 kg ns ha−1 yr−1 in 25 countries in Europe. The average imbalance of ns in nations in the european Economic ar was 12.8 kg ha−1 yr−1 (Runge-Metzger 1995). No country studied confirmed a net loss of P; all nations were p accumulators. In every case, P accumulation was resulted in not through a herbal imbalance in p inputs and also outputs however by entry of fertilizer and animal feeds that surpassed outputs in farming products (Runge-Metzger 1995). Tunney (1990) found an eightfold rise in average obtainable P in the soils of Ireland between 1950 and also 1990. In 1990, p inputs in fertilizer to Ireland were more than double the outputs (Tunney 1990). Isermann (1990) calculated the P surplus (total application of fertilizers minus net withdrawal by farming products) in the Netherlands and also Germany to be 88 and also 63 kg ha−1 yr−1, respectively.
A literature search turned increase eight published attempts come quantify the worldwide P bike (Stumm 1973, Lerman et al. 1975, Pierrou 1976, Richey 1983, Smil 1990, Schlesinger 1991, Jahnke 1992, Reeburgh 1997). These authors quantified the activity of P v 7 come 10 an international pools, including land, soil biota, minable resources, sediments, ocean, and ocean biota. In general, the writer did not focus on P build-up or human impacts. Due to the fact that data for straight calculating a global budget have actually not been conveniently available, and also because over there are many ways to calculate an international pools and fluxes that P, an extensive discrepancies exist. Due to the fact that many fluxes are daunting to measure accurately at the global scale, and because most of this authors were came to with just portions the the global P cycle, some researchers completed fixed balances by assuming an yearly net steady state in the soil compartment. This assumption is debatable, however, as some recent studies display an accretion of ns in soil (Fluck et al. 1992, NRC 1993, Bennett et al. 1999).
Global phosphorus budgets
How has the global P cycle readjusted since the beginning of large-scale mining because that P? We offered a mass-balance method to estimate yearly inputs to, outputs from, and adjust in storage of ns in the surface soils of all terrestrial ecosystems and also all freshwater ecosystems top top the planet (Figure 2). We examined these ns levels for both the current and the preindustrial (before global-scale human being impact) eras. Phosphorous buildup was calculated as inputs minus outputs in both periods. The inputs were P included to surface ar soils and freshwater ecosystems v mining and also weathering, and the outputs to be P removed from this system through net calculation to the atmosphere and also fluvial deliver to the oceans.
We compared the global budgets to a comparable budget calculated for agricultural lands only. Because that the agricultural budget, inputs to be fertilizers and manure; outputs were harvested crops, animals and animal products, and runoff. Us then be separated the agricultural budget outcomes by arising or developed nation status (FAO 2000) to recognize whether the pattern of worldwide P accretion is uniform transparent the world.
Values because that inputs and also outputs for every budgets were established through literature search. Wherever we uncovered ranges of numbers, we chose the much more conservative estimate to minimize current yearly P accretion. Weathering approximates are very variable, so us report a variety of estimates.
Calculating the current worldwide P budget
We calculated annual input that mined ns to surface soils to be about 18.5 Tg yr−1 (1 teragram = 1 million metric tons). Production of phosphate rock in 1995–1996 was 19.8 Tg yr−1; however, not all of this ns enters surface ar soils (FAO 1950–1997). Around 4% that mined ns is supplied to produce commodities such together flame retardants, paper, glass, plastics, rubber, pharmaceuticals, petroleum products, pesticides, and also toothpaste, which are not added to surface soils (RISL 1994). We considered only those assets that readily go into surface soils or freshwater ecosystems together P input—agricultural fertilizers, animal feed supplements, and also detergent builders, for example—and as such did not incorporate the 4% (0.8 Tg yr−1) the phosphate rock the was mined for commercial uses (RISL 1994). Additionally, not all mined ns that is used to produce pet feeds will certainly be integrated into surface ar soils directly: Some will certainly be included into animal tissue. To avoid twin counting, we offered manure manufacturing to calculation input from pet feed to soil. Thus, the lot of mined ns that is included as an intake in our spending plan is 19.8 Tg p mined – 0.8 Tg for industrial uses – 2.0 Tg in pet feeds (RISL 1994) + 1.5 Tg in manure (Isermann 1990) = 18.5 Tg ns yr−1
Global release of p to surface ar soil led to by weathering ranges between a minimum that 15 and a best of 20 Tg yr−1. By combining the mechanical and also chemical denudation prices (20,000 Tg yr−1; Garrels and Mackenzie 1971) v the average P contents of the Earth"s tardy (0.1%; Taylor 1964), Lerman and colleagues (1975) calculated current P weathering to be 20 Tg yr−1. Judson and Ritter (1964) calculated and Gregor (1970) corroborated a pre–human-impact mechanical and chemical denudation price of 10,000 Tg yr−1; this denudation rate says a pre–human-impact weathering rate of 10 Tg yr−1. Because global weathering estimates vary extensively due to differences in methodology or difficulties scaling up from neighborhood studies to come at a global estimate, we use a variety of worths in our budget (Gardner 1990, Newman 1995). Because that the minimal calculation of current weathering rates, we mean the published values for global weathering noted above because that an calculation of 15 Tg of p released per year. Together a preferably estimate, we usage the existing weathering price estimate (20 Tg p yr−1) alone.
Graham and Duce (1979) discovered that 3.2 Tg yr−1 of ns moves from the environment to land and 4.2 Tg yr−1 native the land come the atmosphere. The excess p (1 Tg yr−1) entering the atmosphere from the land—net atmospheric output in our model—is ultimately deposited in the s (Duce et al. 1991).
Modern fluvial ns flux, the lot of ns discharged from the world"s rivers to the ocean, is estimated to it is in 22 Tg yr−1 (Howarth et al. 1995). Fluvial ns flux is calculated as the complete of dissolved and also particulate riverine ns flux. Utilizing data indigenous 20 rivers worldwide, Meybeck (1982) discovered dissolved p flux to it is in 2 Tg yr−1. Comprehensive literature evaluation led Howarth et al. (1995) to estimate particulate p flux come the ocean to be 20 Tg yr−1. This calculation was based on Milliman and Meade"s (1983) estimated total riverine sediment flux the 15*1015 g yr−1 and also an median concentration of p in riverine rely sediment the 1275 mg kg−1 (Martin and also Meybeck 1979, Meybeck 1988). Complete dissolved and also particulate flux to the oceans of 22 Tg yr−1 drops within the published range of 17 Tg yr−1 (Pierrou 1976) come 32 Tg yr−1 (GESAMP 1987).
Inputs of ns to terrestrial soils and also freshwater ecosystems in the current spending plan are in between 33.5 and 38.5 Tg annually. About 23 Tg of ns is calculation annually. Total build-up of p in surface soil and also freshwaters in the modern-day budget was between 10.5 and 15.5 Tg yr−1(Figure 2a). A small over one-quarter of the p input is save on computer in upland soils and freshwaters, follow to this budget.
Calculating a preindustrial global P budget
In the preindustrial budget, weathering entry of ns ranged in between 10 and 15 Tg yr−1. Mining input to be negligible. Together a minimal calculation for preindustrial weathering rates, we usage the released preindustrial weathering prices (Judson and also Ritter 1964, Gregor 1970) to calculate a p weathering rate of 10 Tg yr−1 (see earlier conversation of present weathering price estimates). As a maximal estimate, us assume the the preindustrial and also current weathering prices are identical and also we typical the released values for an international weathering because that an calculation of 15 Tg p released every year.
In the preindustrial budget, p outputs were net atmospheric output and also riverine flux to the oceans. Preindustrial network atmospheric calculation was the exact same as in the existing budget, 1 Tg yr−1 (Graham and Duce 1979). Howarth and colleagues (1995) estimated preindustrial fluvial output of p to the oceans to be 8 Tg yr−1, based upon a dissolved P flux that 1 Tg yr−1 and a particulate flux of 7 Tg yr−1. Data gathered from 20 relatively undisturbed rivers was offered to recognize the 1 Tg yr−1 preindustrial riverine liquified P flux (Meybeck 1982). Riverine particulate flux, 7 Tg p yr−1, was calculated by multiplying the exposed sediment flux, 7 * 1015 g yr−1 (Milliman et al. 1987) by the typical P concentration of this sediments, 1000 mg kg−1 (McKelvey 1973). The lower estimate for suspended sediment p concentration (compared to present estimates) is reasonable due to the fact that it is most likely that farming fertilization has actually increased the p concentration of eroding material (Avnimelech and also McHenry 1984).
Preindustrial budget inputs of ns ranged indigenous 10 come 15 Tg yr−1. About 9 Tg yr−1 is output. Thus, preindustrial P build-up in soil was around 1–6 Tg yr−1(Figure 2b). Preindustrial accumulation was probably variable in time and space, relying on factors such as glaciation and also age of the soil. The overfill P build-up in the modern budget contrasted to the preindustrial spending plan is 4.5 to 14.5 Tg yr−1, which to represent at least a 75% rise in ns storage because preindustrial times.
Our budgets deal with both terrestrial soils and also freshwater ecosystems. While most of the excess ns is probably accumulating in upland soils, some of the overfill P may be accumulating in freshwater sediments. Us calculate the quantity of p accumulating in freshwater sediments to have been between 1 and also 1.2 Tg yr−1 preindustrially and also between 1 and 3.1 Tg yr−1 in ~ the current time. Based on global freshwater area the 10.4 through 1012 m2 (FAO 1998), an median sediment accumulation of 0.1 centimeter yr−1 (Filippelli-Gabriel and also Ruttenberg 1997), and also an typical sediment P content of 3 mg g−1 dry weight (Nürnberg 1988), worldwide P sedimentation in freshwater is at least 1 Tg yr−1. Behrendt (1996) approximated that for the worldwide average hydrologic calculation (0.3 m yr−1; Berner and also Berner <1987>), a 20% p retention in surface ar waters is expected. This argues P retention that 1.2 Tg yr−1 in freshwater ecosystems preindustrially and around 3.1 Tg yr−1 in these equipment now. Phosphorus that is accumulating in freshwaters have the right to be resuspended or mobilized to contribute to downstream eutrophication. Therefore, this p is included in our estimates of P buildup in terrestrial and freshwater sediments globally.
We additionally calculated a global agricultural P spending plan to recognize the quantity of P build-up that wake up in farming areas. This budget included only farming inputs (fertilizer and also manure) and outputs (agricultural products such as meat and eggs, and also runoff). Fertilizer inputs to be calculated based on worldwide estimates the fertilizer use and P contents of fertilizer (FAO 1950–1997). Manure inputs were calculated together in the current worldwide budget. Outputs to be calculated based on agricultural production data (FAO 1950–1998) and also the portion of ns of these commodities (Pierrou 1976). The outcomes presented in figure 3 room budgets calculated because that 1958–1998 in ~ 5-year intervals.
The farming P spending plan indicated that the average yearly P accumulation in farming areas the the people was 8 Tg yr−1 native 1958 to 1998. This figure lies in ~ the variety of overabundance P accumulation in the contemporary budget as contrasted with the preindustrial one. This an outcome suggests that a considerable portion of the excess p in the current global budget is gift stored in agricultural soils, which occupy 11% the the terrestrial area of the planet (World resources Institute 1998).
P accretion occurs in both developed and developing nations, yet these areas show different patterns of P build-up over time (Figure 4). We calculated agricultural P budgets as comprehensive above, but separately because that developing and developed nation status (FAOSTAT farming Data, http://apps.fao.org/page/collections?subset-agriculture). For developed countries, soil p inputs in fertilizer and manure have actually exceeded removed of p from crops and also animal products, resulting in continual buildup of ns in floor over the previous 40 years. For arising nations, p removal in chop yield was higher than ns input and also there was a slight depletion of ns in soils in 1961; through 1996, however, inputs greatly exceeded outputs. The the 8 Tg of p accumulating in farming soils worldwide, about 5 Tg room accumulating in the agricultural lands of arising countries.
Clearly, p is accumulating in Earth"s surface ar soils, primarily in agricultural areas and at a much faster rate than before large-scale mining for p began. There is likewise greater throughput of p in the current budget plan than in the preindustrial estimate. Moreover, P buildup caused by excess fertilizer might be qualitatively various from an increase in ns stock as result of weathering: Fertilizer p input changes both the mass and also the concentration of p in soil, whereas rise in weathering transforms only the complete P mass because it add to both P and other floor constituents. The affect on aquatic ecosystems is thus likely to be different as well, due to the fact that the greater P concentration indigenous fertilizers boosts the flow of p per fixed of floor transported to freshwater. Phosphorus buildup is no much longer a difficulty just in occurred countries; it shows up to it is in of raising importance in occurring nations together well.
Human-caused transforms in the worldwide P budget plan have caused P come accumulate in upland soils, and also greater an international accretion of p in soil may lead to the heightened severity and prevalence of culturally eutrophic waters. Boosting soil ns levels elevate the potential ns runoff to aquatic ecosystems (NRC 1993, USEPA 1996). P is lost from soil in particulate and dissolved forms. Particulate losses, which space the dominant type of loss, occur throughout erosion and runoff events (Sharpley et al. 1992). Liquified losses deserve to be significant in part soils, especially if the stole (Fe), aluminum (Al), and calcium (Ca) absorption capacity of the soil is saturated, allowing P to move an ext readily through the soil towards aquatic ecosystems (Sims et al. 1998). Although the long-term fate of ns that accumulation in soils is unsure (Cassell et al. 1998), the is clean that together soil P content increases, the potential for particulate and dissolved P deliver in runoff boosts (Sharpley et al. 1981, Daniel et al. 1998). Of certain concern is that big amounts of floor P deserve to be mobilized by outstanding precipitation and also erosion occasions or by transforms in land administration practices, such as the conversion of farming land to residential advancement (Daniel et al. 1994, Sharpley et al. 1994).
Because that originates from distributed sources and varies extensively with eco-friendly conditions, nonpoint resource pollution is daunting to measure and also regulate. Policies and also regulations have actually tended to technique P runoff to aquatic ecosystems and also eutrophication together a trouble of the details lake, flow reach, or estuary in question, rather than as component of a bigger pattern. Understanding the global ecological fads behind eutrophication can impact local decisions and also stimulate conversation of large-scale approaches come management.
There room two basic approaches come decreasing the impact of soil p accretion ~ above aquatic ecosystems. We have the right to attempt to carry the P budget into balance by reducing p inputs to soil (controlling sources), and we can shot to reduce the deliver of p from soils to aquatic ecosystems (increasing sinks). In ~ the very same time, it will be important to reduce P concentrations in soils currently overenriched with P due to the fact that of past budget plan imbalances.
Drawdown of floor P could take years or longer in many areas (McCollum 1991, Stigliani et al. 1991). Because of the increase in soil ns concentrations, the threat of eutrophication will be elevated for a lengthy while (Cassell et al. 1998). Over this time period, alters in farm yard practices, metropolitan expansion, or climate change could advice erosion, therefore increasing the price of transfer of p from the soil into aquatic ecosystems. By the time water sources are noticeably impaired, p accretion in terrestrial soils, upstream sediments, rivers, or lakes may currently be an excellent enough to keep high loading to lowland aquatic solution for prolonged periods that time.
Although there are few data ~ above the permanent fate of ns that accumulation in fertilized soils, the slow solution times that Fe–P, Al–P, and Ca–P pools might reduce choices for later management. When dealing with slowly changing variables such together soil p concentrations, mitigation bring away a lengthy time and accumulation costs have the right to be huge (Pizer 1996). By controlling soil ns accretion now, we may have the ability to avoid the prices of eutrophication in the future and also create flexibility for coping v freshwater problems that might arise.
Delivery of p to receiving waters deserve to be decreased not just by reducing p inputs come soils but additionally by raising P sinks in watersheds. Among such sinks are riparian buffers and also wetland areas, detention basins, and also conservation agriculture practices (NRC 1992, Novotny and Olem 1994, Soranno et al. 1996). However, riparian and wetland buffers have a restricted capacity to retain p (Richardson and Qian 1999).
Some locations are at higher risk for increased sediment distribution rates and severity the eutrophication, and these will certainly demand specific management attention. Urban and suburban development—indeed, building and construction projects in general—expedite erosion the P-enriched soil into aquatic ecosystems (Novotny and Olem 1994). Thus, water quality in areas where human population growth is rapid is most likely to decline because that eutrophication. Farming human populations make hefty demands on water supply and freshwater resources, however diminish these solutions by raising eutrophication. Thus, in swiftly urbanizing or suburbanizing areas, particular attention might need to be command to reduce sediment delivery, illustration down floor P, and also balancing the p budgets the surrounding agricultural areas.
Bringing the P budget of farming areas into balance by reducing fertilizer use would reduce P buildup in farming soils, yet doing so may diminish farming output. Worldwide demand for food is suspect to boost as the human population continues to prosper (Daily et al. 1998). Increases in food manufacturing will most likely derive from increased yields from more efficient water use on land currently in production; enhancing production may also require triple the quantity of nitrogen and also P currently in use (Daily et al. 1998, Tilman 1999). Some adjustments in farming practices may allow a palliation in P usage without sacrificing food manufacturing (Frink et al. 1999). Because that example, manure and sewage P could be recycled an ext efficiently, fertilizer can be target to accomplish plant needs at certain times in the crop cycle, and changes in animal production systems might be make (Matson et al. 1998). Suffer in occurred countries says that the rate of P build-up can be diminished even as chop yields rise (Frink et al. 1999, Sharpley and Tunney 2000).
Methods of agricultural production have arisen in response to society"s need for inexpensive, abundant food (Lanyon and Thompson 1996). Pressured to satisfy society"s need for cheap food there is no going out of business, farmer have had to do decisions that have led come specialization and intensification of agricultural production systems. In ~ the very same time, society has taken because that granted a constant supply of cheap, clean water. The two space not compatible uneven soil ns accretion is controlled.
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We give thanks to Paul Ehrlich, Peter Groffman, Sandra Postel, Daniel Schindler, Emily Stanley, and several cotton reviewers because that thoughtful comment on this manuscript. This research was sustained by the Pew foundation and the nationwide Science Foundation.